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So recently my computer has been having a weird problem. Whenever I'm on youtube I get these pop up ads appear whenever I click on something. Not the usual ones that show up next to the video, I mean they just appear in a new window. I can't for the life of me figure out why it's doing this. First, I thought it was just a virus or malware. For virus protection I use:

Malwarebytes Anti-Malware
Windows Security Essentials
Avast!

I ran a boot up scan from Avast. If you don't know that means it runs a scan before your computer fully boots up. Also ran 3 separate full scans with all three of the programs. Picked up a few things here and there, but I still have this problem. Any other ideas on why or what I can do? I use Firefox as my browser btw.

instead of avast, i would use superanti spyware, microsoft security essentials and avast are the same things ( but MSE is better). Also IE9 blocks pop-ups and stuff like that. but that aside get super anti spyware and run all your scans in safe mode and that should fix your problem. also unistal avast. youtube is owned by google so its unlikely you got a virus from youtube